Field of the Invention
The present invention relates to a failure detection device which detects an air-fuel ratio variation among cylinders and catalyst deterioration of an internal combustion engine.
Description of the Related Art
Generally, in a multi-cylinder internal combustion engine, all the cylinders are controlled in the same amount to control the air-fuel ratio. Accordingly, if the fuel injection system of one cylinder fails, for example, the actual air-fuel ratio may vary significantly among the cylinders (hereinafter referred to as an air-fuel ratio variation). Since such a situation leads to deterioration of exhaust emissions, the law requires that engines have a function to detect on-board an air-fuel ratio variation among the cylinders and indicate the failure.
For example, in the multi-cylinder internal combustion engine described in Japanese Patent Laid-Open No. 2009-30455, an air-fuel ratio variation among the cylinders is detected by taking advantage of the characteristic that the amount of hydrogen generated in the combustion chamber increases quadratically as the amount of variation of the air-fuel ratio toward the rich side increases. Even when the total air-fuel ratio after merging of exhaust gas streams from all the cylinders is the same, if a variation toward the rich side occurs in the air-fuel ratio of only one cylinder, the amount of hydrogen generated increases more than when a variation toward the rich side occurs evenly in all the cylinders, and as the oxygen concentration in exhaust gas decreases by the amount of that increase, the output of an LAF sensor on the upstream side of the catalyst varies toward the rich side. By contrast, the output of an O2 sensor on the downstream side of the catalyst assumes a true value free of the influence of hydrogen, since hydrogen is purified in the catalyst.
As a result of main air-fuel ratio feedback control based on the output of the LAF sensor, the output of the LAF sensor assumes a value, which is apparently equivalent to stoichiometry, despite the variation toward the rich side under the influence of hydrogen, while the output of the O2 sensor assumes a value further on the lean side than the stoichiometry which is the true air-fuel ratio. Accordingly, in auxiliary air-fuel ratio feedback control based on the output of the O2 sensor, the learned value of a stoichiometry correction factor is gradually adapted toward the rich side so as to offset the variation toward the lean side of the output of the O2 sensor, and the median value of the output of the LAF sensor is corrected. When that learned value of the stoichiometry correction factor exceeds a predetermined value, it is regarded that an air-fuel ratio variation among the cylinders has occurred, and it is determined that there is a failure.
In a multi-cylinder internal combustion engine, since exhaust gas streams from the cylinders are merged in the exhaust manifold, the length of the exhaust gas circulation path differs among the cylinders, and accordingly the exhaust gas streams hit the LAF sensor to different degrees from one cylinder to another. For this reason, a quick failure determination cannot always be expected even when an air-fuel ratio variation occurs between one cylinder and the other cylinders. To address such cases, the learning sensitivity of the learned value of the stoichiometry correction factor based on the output of the O2 sensor is set to the sensitive side, so that, relative to the same output variation of the O2 sensor, the learned value of the stoichiometry correction factor is increased more rapidly than when the learning sensitivity is set to normal learning sensitivity.
Failure detection of an internal combustion engine includes not only determination of an air-fuel ratio variation among the cylinders as described above but also determination of catalyst deterioration, and determination of that failure is also required by law. Determination of catalyst deterioration takes advantage of a phenomenon that the fluctuation frequency of the exhaust gas air-fuel ratio on the downstream side of the catalyst increases gradually (approaches the fluctuation frequency of the exhaust gas air-fuel ratio on the upstream side) as the catalyst deteriorates, and determination of catalyst deterioration is executed on the basis of an increase in the ratio between the fluctuation frequency of the air-fuel ratio on the downstream side of the catalyst and the fluctuation frequency of the air-fuel ratio on the upstream side (=the fluctuation frequency of the O2 sensor output/the fluctuation frequency of the LAF sensor output).
However, determination of catalyst deterioration cannot be executed properly, if the learning sensitivity of the learned value of the stoichiometry correction factor is set as described above to the sensitive side in order to determine whether or not there is an air-fuel ratio variation among the cylinders.
As the catalyst deteriorates, the output of the O2 sensor on the downstream side of the catalyst tends to remain on the lean side. If the learning sensitivity of the learned value of the stoichiometry correction factor is a normal value, the exhaust gas air-fuel ratio on the downstream side of the catalyst shows a fluctuation state according to the deterioration of the catalyst, so that whether or not the catalyst is deteriorated can be determined without any problem. If the learning sensitivity is set to the sensitive side, however, the median value of the LAF sensor is corrected on the basis of the learned value of the stoichiometry correction factor which has been adapted excessively toward the rich side, so that the actual air-fuel ratio is controlled improperly toward the rich side.
Consequently, the output of the LAF sensor fluctuates near the stoichiometry, while the output of the O2 sensor shifts to the detection limit on the rich side, inevitably causing a significant reduction of the amplitude of the output fluctuation, which makes it difficult to count the fluctuation frequency. Accordingly, the ratio of the fluctuation frequency does not despite the deteriorating catalyst, and proper determination of deterioration can no longer be expected.
That is, if the learning sensitivity of the learned value of the stoichiometry correction factor based on the output of the O2 sensor is set to a normal characteristic, determination of catalyst deterioration is allowed but determination of an air-fuel ratio variation among cylinders is not allowed. Conversely, if the learning sensitivity is set to the sensitive side, determination of an air-fuel ratio variation among cylinders is allowed but determination of catalyst deterioration is not allowed. Thus, determination of catalyst deterioration and determination of an air-fuel ratio variation are placed in a trade-off relation.